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Explorer
Explorer
10,379 Views
Registered: ‎11-23-2013

How to Synchronize clocks between multiple MMCM

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Hello,

We know all the output clocks in one MMCM could keep fixed phase relationship. But how to synchronize clocks between multiple MMCM?

 

For example, clka and clkb are of the same frequency(100MHz) and phase, clka feeds MMCM1 and clkb feeds MMCM2.

clka_o is the output of MMCM1 with frequency equal to 5MHz, and 90 degree phase shift.

clkb_o is the output of MMCM2 with frequency equal to 5MHz, and 45 degree phase shift.

 

If the two MMCM lock at the same time and their post divider begin to divide the VCO clock simultaneously, the synchronization can be achieved. But I doubt that.

 

Does who know how to do?

 

Thank you!

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Scholar austin
Scholar
18,015 Views
Registered: ‎02-27-2008

Re: How to Synchronize clocks between multiple MMCM

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OK,

 

For the non-frequency modified outputs (CLK0), MMCM's driven by the same clock will be in sync.  The CLKDV, CLKFX, are not guaranteed to be in phase (as their state machines may have been reset at different times.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose

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Scholar austin
Scholar
10,333 Views
Registered: ‎02-27-2008

Re: How to Synchronize clocks between multiple MMCM

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c,

 

No need to do that.  Instead, use the required phases from the same MCM.  0, 90, 180, 270, 2x, divide, ALL outputs may be used from the MCM....there is no restricyion to use a sungle port.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Explorer
Explorer
10,311 Views
Registered: ‎11-23-2013

Re: How to Synchronize clocks between multiple MMCM

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Thanks for your answer.

 

I didn't explain my need clearly. In Xilinx's FPGAs, the MMCM's output clocks can be shifted and reconfigured dynamically.  When shift the output clocks, all the output clocks which enable the phase shift attribute will be shifted together.

But in my design, for example, I need two clocks clka and clkb which are synchronous. I want to shift clka and keep clkb unchanged sometimes, and sometimes shift clkb and keep clka.

 

The clock system above could be implemented by dynamically reconfigure the MMCM. I'm just afraid that we may need more such clocks more than one MMCM could generate, and all the clocks should be sync. So I try to use two MMCMs.

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Scholar austin
Scholar
18,016 Views
Registered: ‎02-27-2008

Re: How to Synchronize clocks between multiple MMCM

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OK,

 

For the non-frequency modified outputs (CLK0), MMCM's driven by the same clock will be in sync.  The CLKDV, CLKFX, are not guaranteed to be in phase (as their state machines may have been reset at different times.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose

View solution in original post

Explorer
Explorer
10,291 Views
Registered: ‎11-23-2013

Re: How to Synchronize clocks between multiple MMCM

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Thank you!

 

Though I cannot get as many clocks I need, your answer is still very helpful.

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